1. Field of the Invention
The present invention relates to a magnetic thin film capable of working in a GHz-order frequency band, and to an electronic device comprising it.
2. Description of the Related Technology
With rapid popularization of mobile communication with typically mobile phones, a demand for communication appliances having a high transmission speed is increasing. A novel standard made for increasing the transmission speed of such mobile communication appliances is toward the increase in the working frequency thereof for avoiding a competition with the already-existing electric wave communications technology, and recently, the working frequency of mobile communication appliances has reached a GHz-order level. Accordingly, workability in a GHz band is one indispensable matter for the material to be used for the devices that meet the recent communication standard.
The workability in such a high frequency band is required also for other electronic devices comprising a magnetic material, such as inductor, transformer, etc. However, since a magnetic material increases its loss through ferromagnetic resonance, the devices comprising it are difficult to work in a high-frequency band. For realizing the reduction in the loss of magnetic material in a high-frequency band, it is desirable to increase the saturation magnetization of the material and the anisotropic magnetic field around it to thereby increase the resonance frequency of the material. For obtaining a magnetic material having the characteristics, proposed is a magnetic thin film of a thin magnetic layer and a thin insulating layer alternately laminated on each other. See JP-A-9-063844: JP-A-9-063844 and JP-A-2003-338409.
It is known that the resonance frequency of a magnetic thin film depends on the product of the saturation magnetization and the anisotropic magnetic field thereof, as in the following equation 1 wherein fr indicates the resonance frequency, Ms indicates the saturation magnetization, and Hk indicates the anisotropic magnetic field.
                    fr        =                              γ                          2              ⁢              π                                ⁢                                    Hk              ·              Ms                                                          Equation        ⁢                                  ⁢        1            (In this, γ indicates a gyro magnetic constant.)
A magnetic material has an almost constant saturation magnetization value, and therefore, it has heretofore been considered that the method of increasing the resonance frequency of the material could be attained by actually increasing the anisotropic magnetic filed to thereby increase the magnetization rotation of the material. For the method of increasing the anisotropic magnetic field of a magnetic material, known are (1) thermal treatment of the material in a magnetic film to thereby impart induced magnetic anisotropy thereto, (2) forming a slit through a magnetic thin film to thereby impart profile magnetic anisotropy thereto, and (3) exchange coupling between a ferromagnetic film and an antiferromagnetic film. However, the above are problematic in that the process (1) is complicated as requiring thermal treatment, the process (2) results in substantial reduction in the magnetic permeability of the slit-formed magnetic thin film since the magnetic permeability of the slit part of the film is lost, and the process (3) results in the reduction in the magnetic permeability of the laminate structure since the antiferromagnetic film is formed and the saturation magnetization of the overall magnetic film laminate is thereby lowered.
FIG. 14 is a graph showing the change of the saturation magnetization and the anisotropic magnetic field of a FeCo alloy magnetic material, depending on the Fe proportion in the alloy. From this, it is known that the increase in the saturation magnetization tends to the reduction in the anisotropic magnetic field. As in this, the alloy could hardly satisfy both a high saturation magnetization and a high anisotropic magnetic field, and therefore it is difficult for one and the same kind of a magnetic material to satisfy both a high saturation magnetization and a high anisotropic magnetic field, therefore attaining a reduced loss and an increased resonance frequency in a high-frequency band. In particular, it is extremely difficult to realize the characteristics necessary for obtaining electronic devices that may work in a GHz-level band.